Red Maple, or Acer rubrum, Intoxication

Filed Under: Horses, Poisoning

Are your horses on a poor pasture this fall?  Is there a beautiful red maple tree in the field?  You might want to think twice about this tree being a pasture-mate to your horses.  Green or wilted leaves of the red maple tree can be hazardous to your horse’s health.  

Consumption of the wilted leaves of the red maple will cause an acute and profound Heinz body anemia (destruction of red blood cells) and methemoglobin production.

The toxin involved in the poisoning has, to-date, not been identified.

Heinz bodies have round to oval granules located near the cell margin of a red blood cell.  The Heinz bodies are produced by the precipitation of oxidatively denatured hemoglobin.  Hemoglobin is essential for the transport of oxygen throughout the body.  Mature red blood cells or erythrocytes lack mitochondria within their cells for oxidative metabolism

When the concentration of methemoglobin within a red blood cell becomes greater than 1.5% in concentration, the red blood cell will contain Heinz bodies.  When stained these Heinz bodies appear as bluish-green inclusions within the horse’s red blood cells. 

Methemoglobin is produced when hemoglobin iron is oxidized to the nonfunctional ferric state.  Methemoglobin cannot load or transport oxygen.  Instead of oxygen-rich blood, the blood and mucous membranes in the poisoned horse will appear brown or bluish (cyanotic) in color because methemoglobin is present in sufficient quantities to prevent adequate oxygenation of the tissues.  

Additional clinical signs of red maple poisoning include:  depression, exercise intolerance, jaundice, hemoglobinuria (presence of hemoglobin in the urine), colic, and renal failure.  

Treatment is primarily supportive.  In severe cases a blood transfusion may be of help, but is usually reserved for life-threatening situations.  The use of Vitamin C has been suggested since it may help stop the production of methemoglobin.  Some horses with impaired kidneys may regain renal function with appropriate fluid therapy.


Duncan, Robert and Keith Prasse.  Veterinary Laboratory Medicine Clinical Pathology.  Iowa State University Press. 1977. P .4.

Radostats, Otto and Clive Gay, et al.  Veterinary Medicine.  10th Edition. W.B. Saunders/Elsevier.  2007. P.  1492.

Smith, Bradford.  Large Animal Internal Medicine.  2nd Edition.  1996. Mosby.  Pp. 1227-1228.

Topics: red maple

Symptoms: colic, depression, hemoglobinuria, kidney failure, lethargy

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